The activation of carbon is an old process that has been carried out in many ways. In all the varieties of activation processes now in use the carbon feedstock is a carbonaceous substance from which volatile matter has been substantially removed by heating in the absence of air (charring or carbonization), or from which volatile matter is naturally substantially absent. Examples of the former category are coconut char, wood char ("charcoal"), and bituminous coke. An example of the latter category is anthracite coal of sufficiently low volatile content. The activation is effected by a gasification process that creates a greatly enlarged surface area and an extensive network of submicroscopic pores. The most common gasification processes are: EQU C.sub.solid +H.sub.2 O.sub.gas .fwdarw.CO.sub.gas +H.sub.2.sbsb.gas EQU C.sub.solid +CO.sub.2.sbsb.gas .fwdarw.2CO.sub.gas
These processes require high temperatures (generally above about 700.degree. C.) and are endothermic, and are therefore energy-intensive.
The necessary energy can be supplied from a combustion source, or transferred to the carbon by the superheated steam or carbon dioxide that passes through it, or it can be supplied directly to the carbon by making use of its electrical resistivity. The use of electrical resistance heating offers the advantage of supplying the energy directly to the carbon where it is needed, and there have been various attempts to develop such a process on a practical scale, as described, for example, in U.S. Pat. Nos. 1,634,477, 1,634,478, 1,634,480, 1,686,100, 1,854,387, 1,593,879, 1,597,208, 1,601,222, 1,701,272, 2,003,278, and 2,270,245. However, there have been many difficulties which have impeded or prevented the successful commercial application of electrical resistance heating for manufacture of activated carbon. These difficulties have involved variations of electrical resistivity of the carbons as a function of their source, of their content of volatiles, of temperature, and of other factors. Another difficulty has been the uneven distribution of steam through the carbon granules. The resulting activated carbons have consequently been nonuniform in their properties from batch to batch as well as within each batch. Attempts to bring all parts of a batch of carbon to the desired degree of activation have led to higher energy costs and lower material yields. Attempts to circumvent these difficulties have led to various stratagems such as fluidization of the carbon, or the use of moving parts, in some cases including the electrodes themselves, as exemplified in U.S. Pat. Nos. 1,686,100, 1,593,879, and 1,601,222. We have found that fluidization leads to arcing of electricity between particles and to nonuniform properties. Mechanical actions damage the carbon granules or pellets and reduce the material yield, and necessarily involve higher capital and maintenance costs for the equipment.